Recent progress has been made in developing systems for tympanic membrane temperature measurement. By way of background, mammalian temperature has long been of keen interest to doctors and others involved in diagnosis and treatment of patient pathologies. On the other hand, accurate temperature measurement, accomplished in a quick, unintrusive and inexpensive manner has remained a considerable task. Measuring the temperature of the tympanic membrane of the ear has been found to provide a highly accurate body temperature reading. By collecting the infrared emissions from the tympanic membrane, an accurate temperature reading can be ascertained in an unintrusive procedure.
As stated above, many systems have been proposed for temperature measurement based on tympanic IR emissions. Exemplary patents in this field include U.S. Pat. No. 4,895,164 to Wood, 4,797,840 to Jacob Fraden, Ph.D. and U.S. Pat. No. 5,199,436 to Pompei, et al.; the contents of these patents are incorporated herein by reference. These systems vary in both accuracy and complexity, but in large have been found to be very useful for their intended purposes, and are now enjoying commercial popularity. Notwithstanding these past successes, a common and significant handicap resides with even the most expensive of these systems. This handicap relates to the accuracy and repeatability of the readings obtained.
It has been found that the typical IR thermometer will give a reading that varies in significant amounts depending on the angle and depth of placement of the tip vis-a-vis the ear canal. This variation is caused by changes in the sensor position relative to the wave guide, the ear canal and tympanic membrane. More particularly, the geometric relationship between the sensor and the tympanic membrane will influence the ultimate reading by the sensor in operation. As this geometry changes, the sensor will encounter reading fluctuations independent of actual membrane temperature.
These problems can be better visualized by reference to the prior art probe design and its placement in a typical ear canal--see, e.g., FIGS. 1 and 2. In FIG. 1, a simplified diagram depicts the general elements of an IR type thermometer and its physical relationship with a human ear. In this use, the thermometer develops a field of view of the ear canal and tympanic membrane of the ear as depicted in FIG. 2. As can be appreciated, the field of view of the thermometer will depend on the position in terms of depth and angle as applied by the user of the device.
In this regard, an angular displacement from perpendicular will afford greater influence to the ear canal wall, while a deeper placement of the probe into the ear canal will lessen the influence of the ear canal vis-a-vis the tympanic membrane. Accordingly, different readings will result from the same patient solely as a function of thermometer placement in the ear. Of course, ear canal dimensions will also differ amongst individuals, adding an additional variance. The impact of these variances on typical temperature readings is illustrated in Table I below--which delineates temperature reading as a function of probe position.
TABLE I ______________________________________ Angle Depth Temperature ______________________________________ 0 96.degree. 0 1 mm 97.degree. 20.degree. 0 95.degree. ______________________________________
As noted above, this phenomenon is intimately related to the field of view of the sensor system. This field of view is influenced by several design aspects, chief of these being the relative position of the sensor to the wave guide. Other things being equal, a large diameter wave guide positioned close to a small sensor will exhibit a relatively wide field of view, while a narrow wave guide positioned at a relatively greater distance from a larger sensor (in terms of radiation impingement surface area) exhibits a narrow field of view.
The impact of the field of view for the sensor system can be expressed in the following way. Sensors with a narrow field of view afford accurate readings relatively independent of distance from the target (depth of probe in the ear)--but fluctuate to a greater degree in temperature reading (and accuracy) when angle displacements are introduced in sensor placement vis-a-vis the ear. Alternatively, a sensor with a wide field of view provides reciprocal properties, The wide field of view sensor gives a reading relatively insensitive to angle of probe placement--but is more sensitive to probe depth. These principles may be better visualized by inspection of the graphs in FIGS. 3 and 4.
Although the above-noted dichotomy between narrow and wide views provides a useful illustrative tool, it should be noted that two sensors having the same viewing width may, however, exhibit different views, as the angle of view may be altered. The implication of these properties is that the various geometric sensor arrangements found in prior art IR thermometers are prone to position dependent reading variations.